CN115038803A

CN115038803A - Noble metal tip for spark plug, electrode for spark plug, and spark plug

Info

Publication number: CN115038803A
Application number: CN202180012395.5A
Authority: CN
Inventors: 吉本修; 田中智雄; 服部健吾
Original assignee: NGK Spark Plug Co Ltd
Current assignee: Niterra Co Ltd
Priority date: 2020-02-14
Filing date: 2021-02-02
Publication date: 2022-09-09
Anticipated expiration: 2041-02-02
Also published as: DE112021001025T5; US20230119721A1; JP7350148B2; JPWO2021161845A1; CN115038803B; WO2021161845A1

Abstract

A noble metal tip 32 for a spark plug, which contains not less than 50 mass% of iridium (Ir), not less than 0.1 mass% and not more than 5 mass% of aluminum (Al), and rhodium (Rh), wherein a fibrous microstructure R is observed, the average value of the aspect ratio of the fibrous microstructure R is not less than 150, and the average length in the minor axis direction is not more than 25 [ mu ] m.

Description

Noble metal tip for spark plug, electrode for spark plug, and spark plug

Technical Field

The invention relates to a noble metal tip for a spark plug, an electrode for a spark plug, and a spark plug.

Background

Spark plugs are used as ignition devices for internal combustion engines such as automobile engines. The spark plug has a center electrode and a ground electrode, and a spark discharge is generated by applying a high voltage between these electrodes. Then, the spark discharge ignites the mixed gas. In the electrode of such a spark plug, a tip (ignition portion) mainly made of a noble metal is provided in order to improve ignition performance.

As such a tip, a tip mainly composed of iridium (Ir) having a high melting point is widely used for reasons such as excellent oxidation resistance and wear resistance. However, in recent years, the temperature of the electrode has been increased due to the influence of an increase in temperature and an increase in pressure of the use environment of the engine. Therefore, when the tip is used under a high-temperature atmosphere containing oxygen, iridium is caused to be easily oxidized and volatilized, and a reduction in volume (mass) of the tip becomes a problem.

In view of such circumstances, there has been provided a technique for improving the oxidation resistance of the tip by adding aluminum (Al) to iridium to form a coating (protective film) of an aluminum oxide on the surface of the tip (see patent document 1). The tip is obtained by making an ingot by arc melting an alloy containing iridium and aluminum, and cutting out a predetermined shape from the ingot using a fine cutter.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2008-248322

(problems to be solved by the invention)

There is a large difference between the melting point of iridium (2466 ℃) and that of aluminum (660.3 ℃), and since aluminum has a much lower melting point than iridium, solidification segregation of aluminum is very likely to occur when their alloys are cooled after arc melting. Since the durability of the electrode tip in which solidification segregation occurs is low, when the electrode tip is used as an electrode (ignition portion) of a spark plug, crystal grains may be detached from the electrode tip, and the ignition performance of the spark plug may be lowered.

In addition, when the mixed powder of iridium and aluminum is arc-melted, some powder flying occurs due to the influence thereof. In this case, since aluminum having a small specific gravity is likely to fly, there is a problem that the composition ratio of the mixed powder deviates from a target value, and the performance of the finally obtained tip is unstable.

Disclosure of Invention

The invention aims to provide a noble metal electrode tip for a spark plug and the like with excellent durability.

(means for solving the problems)

Means for solving the problems are as follows. That is to say that the first and second electrodes,

< 1 > a noble metal tip for a spark plug, which comprises 50 mass% or more of iridium (Ir), 0.1 mass% or more and 5 mass% or less of aluminum (Al), and rhodium (Rh), wherein a fibrous metallographic structure is observed in the noble metal tip for a spark plug, the average aspect ratio of the fibrous metallographic structure is 150 or more, and the average length in the minor axis direction is 25 [ mu ] m or less.

< 2 > the noble metal tip for a spark plug according to the above < 1 >, wherein the noble metal tip for a spark plug contains rhodium (Rh) in an amount of 3 mass% or more and less than 30 mass%.

< 3 > the noble metal tip for a spark plug according to the above < 1 > or < 2 >, wherein the noble metal tip for a spark plug comprises at least one of ruthenium (Ru) and nickel (Ni).

< 4 > the noble metal tip for a spark plug as described in the above < 3 >, wherein the noble metal tip for a spark plug contains at least one of ruthenium (Ru) of 3 mass% or more and less than 20 mass% and nickel (Ni) of 0.1 mass% or more and less than 5 mass%.

< 5 > the spark plug noble metal tip as defined in any one of the above < 1 > -4 >, wherein the spark plug noble metal tip has a coating film containing an aluminum oxide on a surface thereof.

< 6 > an electrode for a spark plug, wherein the electrode for a spark plug has the noble metal tip for a spark plug according to any one of the above < 1 > -5 >.

< 7 > a spark plug, wherein said spark plug has said electrode for spark plug < 6 >.

< 8 > a spark plug having the spark plug noble metal tip as defined in < 5 > at least one of a center electrode and a ground electrode, wherein the coating film is provided at least on a discharge surface of the spark plug noble metal tip.

Effects of the invention

According to the present invention, a noble metal tip for a spark plug and the like having excellent durability can be provided.

Drawings

Fig. 1 is a partially cross-sectional explanatory view of a spark plug according to a first embodiment.

FIG. 2 is a perspective view of the electrode head.

Fig. 3 is an explanatory view schematically showing a fibrous metallographic structure contained in the tip.

FIG. 4 is an explanatory view schematically showing a method of manufacturing the tip.

Fig. 5 is a cross-sectional view schematically showing the structure of an electrode tip having a coating formed thereon.

Fig. 6 is an explanatory view schematically showing a metallographic structure contained in the tip of comparative example 2.

Fig. 7 is a view obtained by visualizing the distribution of aluminum by EDS elemental mapping in an SEM image of a cut surface in the vicinity of the electrode tip surface in example 14.

Fig. 8 is a view obtained by visualizing the distribution of oxygen by EDS elemental mapping in an SEM image of a cut surface in the vicinity of the electrode tip surface in example 14.

Detailed Description

< first embodiment >

A first embodiment of the present invention will be briefly described with reference to fig. 1 to 5. In the present embodiment, the spark plug 1 is exemplified together with an electrode for a spark plug and a noble metal tip for a spark plug used therein.

Fig. 1 is a partially sectional explanatory view of a spark plug 1 of the first embodiment. A straight line (one-dot chain line) extending in the vertical direction shown in fig. 1 indicates the axis AX of the spark plug 1. The front end side of the spark plug 1 is arranged on the lower side of fig. 1, and the lower end side of the spark plug 1 is arranged on the upper side of fig. 1. In fig. 1, the portion from the axis AX to the right side shows the appearance of the spark plug 1, and the portion from the axis AX to the left side shows a cross-sectional view of the spark plug 1.

The spark plug 1 is mounted on an engine (an example of an internal combustion engine) of an automobile and is used to ignite a gas mixture in a combustion chamber of the engine. The spark plug 1 mainly has an insulator 2, a center electrode 3, a ground electrode 4, a terminal fitting 5, a metal fitting 6, a resistor 7, and sealing members 8, 9.

The insulator 2 is a substantially cylindrical member extending in the vertical direction and including a through hole 21 therein, and is made of ceramic such as alumina.

The metallic shell 6 is a member used when the spark plug 1 is mounted to an engine (specifically, an engine cylinder head), has a cylindrical shape extending in the vertical direction as a whole, and is made of a conductive metal material (for example, a low carbon steel material). A screw portion 61 is formed on the outer surface of the metal shell 6 on the distal end side. Further, a seat portion 62 is formed at the rear end side of the screw portion 61 so as to protrude outward in an annular shape. An annular shim G is externally fitted to a rear end (so-called screw head) of the screw portion 61. Further, a tool engagement portion 63 is provided on the rear end side of the metal shell 6, and the tool engagement portion 63 is used for engaging a tool such as a wrench when the metal shell 6 is mounted on an engine. A fastening portion 64 bent inward in the radial direction is provided at the rear end portion of the metal shell 6.

The metal shell 6 has a through hole 65 penetrating therethrough in the vertical direction, and the insulator 2 is held inside the metal shell 6 so as to be inserted through the through hole 65. The rear end of the insulator 2 is in a state of protruding greatly outward (upward in fig. 1) from the rear end of the metallic shell 6. In contrast, the front end of the insulator 2 slightly protrudes outward (downward in fig. 1) from the front end of the metallic shell 6.

The center electrode 3 is disposed inside the insulator 2 in a state of being mounted inside the metallic shell 6. The center electrode (an example of an electrode for a spark plug) 3 includes a rod-shaped center electrode main body 31 extending in the vertical direction and a columnar (disk-shaped) electrode tip (ignition portion) 32 attached to the tip of the center electrode main body 31. The center electrode body 31 is a member having a length shorter in the longitudinal direction than the insulator 2 and the metallic shell 6, and is held in the through hole 21 of the insulator 2 so that the distal end side thereof is exposed to the outside. The rear end of the center electrode main body 31 is housed inside the insulator 2. The center electrode main body 31 is made of nickel (Ni) or a nickel-based alloy containing the most nickel (for example, NCF600, NCF601, or the like). The center electrode body 31 may have a double-layer structure including a sheath portion (base material) made of nickel or a nickel-based alloy and a core portion embedded in the sheath portion. In this case, the core portion is preferably formed of copper (Cu) or a copper-based alloy containing the most copper, which is superior in thermal conductivity to the sheath portion. The electrode tip 32 of the center electrode 3 will be described in detail later.

The terminal fitting 5 is a rod-shaped member extending in the vertical direction, and is attached so as to be inserted into the rear end side of the through hole 21 of the insulator 2. The terminal fitting 5 is disposed on the rear end side of the center electrode 3 in the insulator 2 (through hole 21). The terminal fitting 5 is made of a conductive metal material (for example, low carbon steel). The surface of the terminal fitting 5 may be plated with nickel or the like for the purpose of corrosion prevention or the like.

The terminal fitting 5 includes a rod-shaped leg portion 51 disposed on the front end side, a flange portion 52 disposed on the rear end side of the leg portion 51, and a cap mounting portion 53 disposed on the rear end side of the flange portion 52. The leg 51 is inserted into the through hole 21 of the insulator 2. The flange portion 52 is a portion that is exposed from the rear end portion of the insulator 2 and locked to the rear end portion. The cap mounting portion 53 is a portion to which a plug cap (not shown) of a high-voltage cable is mounted and connected, and a high voltage for generating spark discharge is applied from the outside through the cap mounting portion 53.

The resistor 7 is disposed between the front end of the terminal fitting 5 (the front end of the leg 51) and the rear end of the center electrode 3 (the rear end of the center electrode main body 31) in the through hole 21 of the insulator 2. The resistor 7 has a resistance value of, for example, 1k Ω or more (e.g., 5k Ω), and has a function of reducing radio noise when spark occurs. The resistor 7 is made of a composition containing, for example, glass particles as a main component, ceramic particles other than glass, and a conductive material.

A gap is provided between the front end of the resistor 7 and the rear end of the center electrode 3 in the through hole 21, and a conductive sealing member 8 is disposed so as to fill the gap. A gap is also provided between the rear end of the resistor 7 and the front end of the terminal fitting 5 in the through hole 21, and a conductive sealing member 9 is disposed so as to fill the gap. Each sealing member 8, 9 is composed of, for example, B ₂ O ₃ -SiO ₂ Glass particles such as glass particles and metal particles (Cu, Fe, etc.).

The ground electrode 4 is formed as a whole by a plate piece bent in a substantially L shape in the middle, and the rear end portion 42 thereof is joined to the front end of the metallic shell 6. The distal end portion 41 is disposed so as to face the distal end portion (the electrode tip 32) of the center electrode 3 with a space therebetween. The ground electrode 4 and the metal shell 6 are joined to each other by a welding technique such as resistance welding or laser welding. Thereby, the ground electrode 4 and the metallic shell 6 are electrically connected to each other. The ground electrode 4 is made of nickel or a nickel-based alloy, for example, as in the metal shell 6.

A gap S exists between the tip 32 located at the leading end portion of the center electrode 3 and the leading end portion 41 of the ground electrode 4, and when a high voltage is applied between the center electrode 3 and the ground electrode 4, spark discharge is generated in the gap S in a form substantially along the axis AX.

Next, the electrode tip 32 will be described in detail. Fig. 2 is a perspective view of the electrode head 32. The tip (an example of a noble metal tip for a spark plug) 32 is a member attached to the tip of the center electrode 3 as an ignition portion, and has a columnar shape (disk shape). The upper surface 32a and the lower surface of the tip 32 are circular, and the upper surface 32a is mounted in contact with the lower end surface of the rod-shaped center electrode body 31. The electrode tip 32 and the center electrode body 31 are joined to each other by a welding technique such as resistance welding, laser welding, or the like.

The tip 32 is made of an iridium-based alloy containing iridium (Ir) as a main component and containing aluminum (Al) as another component. Specifically, the iridium-based alloy includes 50 mass% or more of iridium (Ir), 0.1 mass% or more and 5 mass% or less of aluminum (Al), and rhodium (Rh).

In addition, a fibrous metallographic structure was observed in the tip 32 made of such an iridium-based alloy.

Fig. 3 is an explanatory diagram schematically showing a fibrous metallographic structure contained in the electrode tip 32. Fig. 3 shows a microstructure R of an iridium-based alloy elongated in a fiber shape in the left-right direction. In the present specification, a fibrous metallographic structure made of an iridium-based alloy may be referred to as a "fibrous structure". The fibrous structure R is formed by stretching at the time of hot working in a method of manufacturing the electrode tip 32 described later. The double-headed arrow a shown in fig. 2 and 3 indicates the longitudinal direction of the fibrous structure R (i.e., the extending direction of the fibrous structure R). The tip 32 is provided so that the longitudinal direction (extending direction, double arrow a) of the fibrous structure R coincides with the direction of the axis AX of the spark plug 1 (in other words, is parallel).

The aspect ratio of the fibrous structure (crystal grain) R is determined by the following method. First, the tip 32 is cut on a surface including the axis AX of the spark plug 1, and the cut surface is ground to obtain a ground surface. Fig. 3 shows a cut surface (ground surface) of the tip 32 cut from a plane including the axis AX (the direction of the double arrow a). Then, the polished surface was observed by an FE-SEM (Field Emission Scanning Electron Microscope), and the maximum length l of the fibrous structure (crystal grain) R in the direction parallel to the axis AX (the direction of the double arrow A shown in FIG. 3) and the maximum length m of the fibrous structure (crystal grain) in the direction perpendicular to the axis AX (the direction of the double arrow B shown in FIG. 3) were measured. Similarly, the maximum length L and the maximum length M are measured for each of the plurality of fibrous structures R, L/M is calculated for each of the fibrous structures R, and the average value (for example, the average value of 20 crystal grains) L/M of the calculated values is defined as the aspect ratio of the fibrous structure (crystal grain) R. One (M) having a smaller median value of the maximum lengths L, M (average value) is an average length of the fibrous structure (crystal grain) R in the short axis direction. The average length of the fibrous structure (crystal grains) R in the major axis direction is L.

In addition, in the tip 32 made of the iridium-based alloy, the average value L/M of the aspect ratio of the fibrous metallographic structure is 150 or more, and the average length M in the short axis direction is 25 μ M or less. When the fibrous metallographic structure has an aspect ratio (average value) and an average length in the short axis direction in such ranges, the crystal grains are suppressed from falling off from the tip 32, and the durability is excellent.

The aspect ratio (average value) is preferably 160 or more. The average length M in the short axis direction is preferably 14 μ M or more, and preferably 19 μ M or less.

In the iridium-based alloy used for the tip 32, the content ratio (lower limit value) of iridium (Ir) is preferably 55% by mass or more, and more preferably 60% by mass or more.

For example, the iridium-based alloy used in the tip 32 may contain 50 mass% or more of iridium (Ir), 0.1 mass% or more and 5 mass% or less of aluminum (Al), and 3 mass% or more and less than 30 mass% of rhodium (Rh). When the content of aluminum (Al) in the iridium-based alloy is within the above range, workability, durability, and the like are excellent. When the content of rhodium (Rh) in the iridium-based alloy is within the above range, workability, durability, and the like are excellent.

The iridium-based alloy may further contain at least one of ruthenium (Ru) and nickel (Ni). In this case, the iridium-based alloy may contain at least one of ruthenium (Ru) of 3% by mass or more and less than 20% by mass and nickel (Ni) of 0.1% by mass or more and less than 5% by mass. When the content of ruthenium (Ru) in the iridium-based alloy is within the above range, workability, durability, and the like are excellent. When the content of nickel (Ni) in the iridium-based alloy is within the above range, workability, durability, and the like are excellent.

In the present embodiment, ruthenium (Ru) and nickel (Ni) are optional components, and are incorporated in the iridium-based alloy as needed.

The iridium-based alloy may contain other elements such as platinum (Pt) as optional components as long as the object of the present invention is not impaired.

Next, a method for manufacturing the electrode tip 32 will be described with reference to fig. 4. Fig. 4 is an explanatory view schematically showing a method of manufacturing the electrode tip 32. As shown in fig. 4(a), first, a raw material powder P having iridium as a main component and a predetermined composition ratio is prepared. The raw material powder P is a mixed powder of iridium powder, aluminum powder, rhodium powder, and the like, and each component is blended so as to have the above composition ratio. The particle diameter of each powder is substantially the same as the particle diameter of the raw material powder used in the production of the electrode tip. In this manner, the raw material powder P having a uniform composition can be obtained by mixing the respective components in a powder state.

Next, as shown in fig. 4(b), the raw material powder P is press-molded into a predetermined shape (for example, a cylindrical shape) by a predetermined powder press, thereby obtaining a molded body 100. By press-molding (powder press-molding) the molded body 100 in this manner, the molded body 100 having a uniform composition can be obtained. Here, a columnar molded body 100 was obtained.

Then, the obtained compact 100 is melted by arc melting and hot forged to obtain an ingot 110 as shown in fig. 4 (c).

After obtaining the ingot 110, the ingot 110 is hot worked while maintaining a temperature of red heat so that aluminum is not segregated by a decrease in temperature. For example, the obtained columnar ingot 110 is elongated in one direction by hot rotary forging (so-called hot swaging) using a rotary hammer, hot wire rolling (for example, hot wire rolling using grooved rolls with a roll pass) or a combination thereof to prepare a rod-like material, and the rod-like material is further elongated in one direction by hot wire drawing using, for example, a wire-drawing die to obtain a linear material 200 as shown in fig. 4 (d). In this manner, the ingot 110 is stretched in one direction by the hot working, thereby forming the wire-like material 200. The linear material 200 is in the form of a long and narrow elongated cylinder, and has a circular cross section (a cross section perpendicular to the direction of elongation). The double-headed arrow C in fig. 4(d) indicates the extending direction of the linear material 200.

As shown in fig. 4(e), the wire-like material 200 is cut at predetermined intervals in the extending direction (longitudinal direction) (i.e., cut in a direction perpendicular to the extending direction), thereby obtaining the electrode tip 32. Such a tip 32 has a fibrous metallographic structure (fibrous structure) R (see fig. 3) elongated in the extending direction C. As described above, the electrode tip 32 can be manufactured from the raw material powder P.

As described above, after the wire-shaped material 200 is cut at predetermined intervals, the resultant tip 32 may be subjected to a heat treatment under predetermined high-temperature conditions in an oxidizing atmosphere (i.e., an atmosphere containing a large amount of an oxidizing gas such as oxygen), thereby forming the coating 32x on the surface of the tip 32. The heat treatment may be performed in an oxidizing atmosphere, for example, in an atmospheric atmosphere, or in an atmosphere in which an oxidizing gas is actively supplied from the outside. The high temperature conditions for the heat treatment include, for example, a temperature range of 800 to 950 ℃.

Fig. 5 is a cross-sectional view schematically showing the structure of the electrode tip 32 on which the coating 32x is formed. Fig. 5 schematically shows a state where the coating 32x is formed so as to cover the entire surface of the inner portion 32y of the chip 32. The coating 32x comprises primarily aluminum oxide and is typically about 1 μm to about 10 μm thick. In the present specification, "aluminum oxide" refers to a substance obtained by oxidizing aluminum (i.e., an oxide of aluminum), and may be, for example, Al ₂ O ₃ From Al ₂ O ₃ An oxide of aluminum represented by a chemical formula other than the above.

The tip 32 before heat treatment (i.e., the tip obtained by cutting the wire-shaped material 200 into a tip shape) contains, in addition to aluminum (Al), other metal elements such as iridium (Ir) and rhodium (Rh), but aluminum reacts more readily with oxygen than such other metal elements (the metal elements used in the tip 32). Therefore, it is presumed that a coating film mainly containing an aluminum oxide is formed on the surface of the electrode tip 32 by the heat treatment. In the electrode tip 32, the inner portion 32y covered with the coating film 32x does not substantially contain aluminum oxide (oxygen). It is estimated that aluminum is present in a non-oxide (specifically, metallic aluminum) state other than an oxide in the inner portion 32 y.

As described above, when the coating 32x including the aluminum oxide is formed on the surface of the chip 32, the iridium (particularly, iridium in the vicinity of the surface) present in the chip 32 (specifically, in the inner portion 32 y) is protected by the coating 32x, and the volatilization and oxidation of iridium (Ir) are suppressed. As a result, the durability of the tip 32 is further improved. When aluminum oxide is present up to the inner portion 32y, grain boundary cracks may occur due to volume expansion under high temperature conditions (for example, 1100 to 1200 ℃). Therefore, the inner portion 32y is preferably made of aluminum metal.

The presence of the coating 32X containing an aluminum oxide can be confirmed by, for example, a scanning electron microscope (SEM-EDS) equipped with an energy dispersive X-ray analyzer. The fibrous metallographic structure described above is observed in the inner portion 32y of the tip 32.

As shown in fig. 5, when the upper surface 32a and the lower surface 32b of the tip 32 are mounted so that the upper surface 32a contacts the lower end surface of the rod-shaped center electrode body 31 (see fig. 1), the lower surface 32b serves as a discharge surface of the spark plug. In the tip 32 for the center electrode 3, a surface facing the ground electrode 4 serves as a discharge surface of the center electrode 3. Therefore, the electrode tip 32 is preferably formed with a coating 32x at least in a portion (lower surface 32b) to be a discharge surface. In addition, the heat treatment for forming the coating 32x on the tip 32 may be performed in a state where the tip 32 is mounted on the center electrode body 31 as long as the object of the present invention is not impaired.

In the present embodiment, when the tip 32 is manufactured, the respective components of iridium, aluminum, and the like as raw materials are uniformly mixed in a powder state, and the formed body 100 is manufactured while maintaining the obtained raw material powder P in a uniformly mixed state. Therefore, it is suppressed that aluminum or the like having a small specific gravity is removed from the raw material powder P by flying during the production process, and the composition of the raw material powder P is changed.

In the present embodiment, since the ingot 110 obtained from the compact 100 is directly drawn in one direction by hot working in a red hot state, a predetermined fibrous metallographic structure R is obtained in a state in which solidification segregation of alumina or the like is suppressed in the interior of the ingot 110 (i.e., the wire-shaped material 200) after drawing. The tip 32 cut out from the wire-shaped material 200 has a fibrous metallographic structure R made of a predetermined iridium-based alloy therein, and therefore has no granular crystal grains that are easily detached, and is excellent in durability.

< other embodiments >

In another embodiment, for example, a tip made of the same material as the tip 32 may be attached to the leading end portion 41 of the ground electrode 4 shown in fig. 1 so as to face the tip 32. The tip for the ground electrode 4 is provided so that the longitudinal direction (extending direction) of the fibrous structure coincides with the axis AX direction (in other words, is parallel) as in the tip 32 for the center electrode 3 of the first embodiment. The tip for the ground electrode 4 is less likely to cause the crystal grains to fall off, and has excellent durability. A coating containing an aluminum oxide may be formed on the surface of the tip for the ground electrode 4 in the same manner as in the case of the center electrode. In this case, it is preferable that the tip for the ground electrode 4 has a coating formed on at least a surface (discharge surface) facing the center electrode 3.

The present invention will be described in more detail below with reference to examples. It should be noted that the present invention is not limited to these examples.

[ examples 1 to 15]

The raw material powders of examples 1 to 15, which used iridium (Ir) as a main raw material, were prepared in the composition ratios (mass%) shown in table 1. From the obtained raw material powder, the tip of each example was produced in the same manner as in the above-described method for producing a tip (see fig. 4). Specifically, the raw material powder is subjected to powder press molding to prepare a compact, the obtained compact is melted by arc melting, and an ingot is obtained by hot forging. The obtained ingot was directly subjected to hot working in a red-hot state to obtain an elongated linear columnar linear material extending in one direction. Then, by appropriately cutting the wire-like material, a cylindrical electrode tip (size: diameter 0.8mm, thickness 0.6mm) was obtained.

Comparative examples 1 to 3

The raw material powders of comparative examples 1 to 3 were prepared in the composition ratios (mass%) shown in table 1. The tip of comparative example 1 was produced from the obtained raw material powder by the same method as in example 1 and the like.

In comparative example 3, the iridium-based alloy was hard to process due to excessive hardness, and the ingot was broken when the iridium-based alloy was processed into a wire-like material by hot working. Thus, for comparative example 3, the production of the electrode tip was abandoned halfway.

In comparative example 2, unlike example 1 and the like, an alloy containing iridium and aluminum was arc-melted to prepare an ingot, and the obtained ingot was subjected to cutting processing to obtain a tip of comparative example 2. The external shape (size) of the tip of comparative example 2 was the same as that of example 1 and the like.

[ State of metallographic Structure ]

For the electrode tips of the respective examples and the like, the internal metallographic structure was observed. Specifically, the electrode tip was cut at a surface including the extending direction (axial direction of the spark plug), and the polished surface obtained by polishing the cut surface was observed by FE-SEM. The results are shown in Table 1. In table 1, the metallic structure is represented as "linear" when a fibrous metallic structure is observed, and is represented as "granular" when a granular metallic structure is observed.

[ aspect ratio, etc. ]

The aspect ratio of the metallographic structure was determined for each of the electrode tips of examples and the like. Specifically, the average value (L/M) of the aspect ratio among the total 20 metallic structures (crystal grains) was obtained for the electrode tips of the respective examples and the like. L is an average length of the metallic structure in the major axis direction, and M is an average length of the metallic structure in the minor axis direction. L, M are described above. Table 1 shows the aspect ratio and the average length M in the short axis direction of each example and the like.

[ evaluation test for grain exfoliation ]

Spark plug test pieces were produced using the electrode tips of the respective examples and the like. The electrode tip serves as an ignition portion of a center electrode of the spark plug test body. The basic configuration of the spark plug test piece is the same as that of the spark plug of the first embodiment.

A coating containing aluminum oxide is formed on the surface (discharge surface, etc.) of the electrode tip of each example or the like used as the center electrode (ignition portion) of the spark plug test body. The heat treatment for forming the coating film is performed together with the heat treatment at the time of forming the sealing member (corresponding to the sealing member 8 of the first embodiment) of the spark plug test body. The heat treatment for forming the coating film is described below.

The sealing member is formed by mixing B ₂ O ₃ -SiO ₂ Glass particles such as glass particles, and a conductive glass powder mixture such as metal powder (e.g., Cu or Fe) by sintering. The glass powder mixture is compressed and filled into a through hole (through hole 21) of the cylindrical insulator (insulator 2) held inside the metal shell (metal shell 6) and having the center electrode (center electrode 3) of the tip welded thereto inserted, and a resistor composition for forming the resistor (resistor 7) is further filled in a stacked manner into the glass powder mixture. The resistor composition is prepared by mixing conductive carbon black, ceramic particles and a predetermined binder, respectively, and mixing water as a medium, and then drying the slurry obtained by mixing, and then drying glass powder (for example, a glass powder prepared from B) ₂ O ₃ -SiO ₂ Class of glass material)Mixed therein and stirred. Next, a high heat-resistant press pin having a release agent adhered to the tip end portion thereof is inserted into the through hole of the insulator, and then, in a state where the press pin is pressed into the through hole of the insulator from the opposite side of the center electrode, a treatment (heat treatment) of heating the tip such as a glass powder mixture is performed in a firing furnace under a high temperature condition (800 to 950 ℃) for oxidizing aluminum on the tip surface at a glass transition temperature or higher for a predetermined time (for example, about 20 minutes) in an oxidizing atmosphere. Then, the electrode tip is naturally cooled while the press pin is kept pressed in, thereby forming a sealing member and a resistor, and forming a coating film on the surface of the electrode tip.

The obtained spark plug test body was mounted on a supercharged engine for testing, and a test was performed by operating the engine for 200 hours while keeping the air-fuel ratio (air/fuel) of the air-fuel mixture at 14, the throttle valve fully open, and the engine speed at 6000 rpm. The ignition angle of the spark plug test piece during engine operation was set to BTDC35 °, and the intake pressure was set to-30 KPa. After the test, the spark plug test piece was removed from the engine, and the tip of the spark plug test piece was observed with a magnifying glass to confirm whether or not the crystal grains fell off. The results are shown in Table 1. In table 1, the case where there is crystal grain shedding is represented as "presence", and the case where there is no crystal grain shedding is represented as "absence".

[ durability evaluation test ]

A spark plug test piece was produced independently of the spark plug test piece used in the crystal grain detachment evaluation test using the electrode tip of each example and the like. Then, the spark plug test piece was set in a pressurizing chamber, and the test using the discharge of the spark plug test piece was repeated under the conditions of 100Hz and 3 hours in a nitrogen atmosphere pressurized to 0.6 MPa. The mass change before and after the test was determined for the tip of the spark plug test piece used in the test, and the value obtained by dividing the change amount (g) by the tip density determined in advance before the test was determined as the consumed volume.

And, the consumption volume is 0.05mm ³ In the above case, it was judged that the wear was severe and there was no durability, and it is indicated by "x" in table 1.

In addition, the consumption volume was 0.04mm ³ Above and less than 0.05mm ³ In the case of (1), the consumption was judged to be low and the durability was observed, and the result is indicated as "o" in table 1.

In addition, the consumption volume was 0.03mm ³ Above and less than 0.04mm ³ In the case of (2), the durability was judged to be more excellent, and is shown to be "good +" in table 1.

In addition, the consumption volume is less than 0.03mm ³ In the case (2), the durability was judged to be particularly excellent, and is shown to be "good +", in table 1.

[ Table 1]

As shown in table 1, the tips of examples 1 to 15 each were made of an iridium-based alloy, and a fibrous metallographic structure was observed on the cut surface (ground surface) thereof, and the average aspect ratio (L/M) of the metallographic structure was 150 or more, and the average length M in the short axis direction was 25 μ M or less. It was confirmed that such an electrode tip suppressed the falling of crystal grains and was excellent in durability.

The tip of comparative example 1 contains less than 0.1 mass% of aluminum. The electrode tip of comparative example 1 had poor durability as a result of an excessively low aluminum content.

The tip of comparative example 2 had a granular metallurgical structure. Fig. 6 is an explanatory view schematically showing a metallographic structure contained in the electrode tip of comparative example 2. In comparative example 2, the ingot was cut to obtain a tip. Therefore, in the tip of comparative example 2, a metallographic structure was observed which was composed of granular crystal grains X having a small aspect ratio. From the results of the crystal grain detachment evaluation test, it was confirmed that in such an electrode tip, the crystal grains X easily detached.

Comparative example 3 is a case where the aluminum content ratio in the iridium-based alloy is high. As described above, in comparative example 3, workability was difficult due to excessive hardness of the iridium-based alloy.

As shown in Table 1, the durability of examples 9 to 15 is more excellent than that of examples 1 to 8 in examples 1 to 15, and among them, the durability of examples 13 to 15 is confirmed to be particularly excellent.

[ confirmation of coating film ]

Here, as a representative example, the coating formed on the surface (discharge surface) of the electrode tip in example 14 was confirmed by SEM EDS. The results are shown in fig. 7 and 8. Fig. 7 is a view obtained by visualizing the distribution of aluminum by EDS elemental mapping in an SEM image of a cut surface in the vicinity of the electrode tip surface in example 14. As shown in fig. 7, aluminum is uniformly dispersed in the electrode head as a whole. That is, aluminum is uniformly dispersed not only in the coating 32x portion of the surface layer but also in the inner portion 32y located inside the coating 32 x. Note that symbol S10 shown in fig. 7 indicates a space (the same applies to fig. 8).

Fig. 8 is a view obtained by visualizing the distribution of oxygen by EDS elemental mapping in an SEM image of a cut surface in the vicinity of the electrode tip surface in example 14. As shown in fig. 8, oxygen is present only in the coating 32x portion of the surface layer, but not in the inner side portion 32 y. In this way, since oxygen is present in the surface layer, it can be said that the coating film 32x containing aluminum oxide is formed.

As shown in fig. 8, oxygen is observed only in the surface layer (coating film 32x) and is not observed in the inner portion 32y, and therefore, it can be said that the inner portion 32y does not contain aluminum oxide. As described above, when only the coating film 32x of the surface layer contains aluminum oxide and the inner portion 32y does not contain aluminum oxide, the problem that the tip expands in volume at a high temperature to cause grain boundary cracks or the like is suppressed.

Description of the symbols

1 … spark plug, 2 … insulator, 3 … center electrode (electrode for spark plug), 31 … center electrode body, 32 … tip (noble metal tip for spark plug), 4 … ground electrode, 5 … terminal fitting, 6 … body fitting, 7 … resistor body, 8, 9 … sealing member

Claims

1. A noble metal tip for a spark plug, which contains 50 mass% or more of iridium (Ir), 0.1 mass% or more and 5 mass% or less of aluminum (Al), rhodium (Rh), and

a fibrous metallographic structure is observed in the spark plug noble metal tip, and the fibrous metallographic structure has an average aspect ratio of 150 or more and an average length in the minor axis direction of 25 [ mu ] m or less.

2. A noble metal tip for a spark plug as set forth in claim 1, wherein said noble metal tip for a spark plug contains 3% by mass or more and less than 30% by mass of rhodium (Rh).

3. The noble metal tip for a spark plug according to claim 1 or claim 2, wherein the noble metal tip for a spark plug contains at least one of ruthenium (Ru) and nickel (Ni).

4. A noble metal tip for a spark plug according to claim 3, wherein said noble metal tip for a spark plug comprises at least one of ruthenium (Ru) of 3 mass% or more and less than 20 mass% and nickel (Ni) of 0.1 mass% or more and less than 5 mass%.

5. The spark plug noble metal tip according to any one of claims 1 to 4, wherein the spark plug noble metal tip has a coating film containing aluminum oxide on a surface thereof.

6. An electrode for a spark plug, comprising the noble metal tip for a spark plug according to any one of claims 1 to 5.

7. A spark plug having the electrode for a spark plug according to claim 6.

8. A spark plug having at least one of a center electrode and a ground electrode of the spark plug a noble metal tip for a spark plug according to claim 5,

the coating film is provided at least on the discharge surface of the spark plug noble metal tip.